Pentagon Sees 'Critical Challenge' in N.Korean Missiles

North Korea has been increasing its capability to strike the U.S. mainland, U.S. Defense Secretary Chuck Hagel warned in an annual report to Congress on military and security developments involving the renegade state.

Hagel delivered the report on Thursday under the National Defense Authorization Act.

The North proved "its need to counter perceived U.S. 'hostility' with nuclear-armed [intercontinental ballistic missiles]. North Korea will move closer to this goal, as well as increase the threat it poses to U.S. forces and allies in the region," the report says. The North also "continues to invest in its nuclear infrastructure."

But the report is vague about when the North will be capable of striking the U.S. mainland. It cites a rocket launch last year that is widely believed to have been a test of long-range missile technology.

"However, a space launch does not test a re-entry vehicle (RV), without which North Korea cannot deliver a weapon to target from an ICBM," the report adds.